Optical Coherence Tomography (OCT) systems are based on Michelson interferometers. Light from a low optical coherence (i.e. broadband) source is split into two arms. The length of one arm (the reference arm) is defined by a mirror. On the other arm the mirror is replaced by the sample that backscatters light into the interferometer. The light from the two arms is recombined, and only light that has been backscattered at a depth that matches the length of the reference arm within the coherence length of the source can interfere. Such coherence length is defined by the source spectral width, and is typically a few microns when the optical bandwidth is a few tens of nanometers.
By altering the optical length of the reference arm, for example by varying the reference mirror position, it is possible to explore scattering at different sample depths. In practice, the sample surface is typically scanned sequentially (e.g., by raster or conical scan), or in parallel (using an array of detectors), and the sample depth is probed by scanning the reference arm optical length (for example, by a mechanical scan), exploring a typical depth on the order of 2 mm. An alternative technique for depth scanning relies on the fact that the interferogram collected by scanning the optical length of the reference arm is effectively the Fourier transform of the spectrum collected on the interferometer output. Therefore, the reference arm can be kept fixed, and the interferometer output is connected to a spectrograph or a frequency-swept narrowband source is used to explore the spectrum sequentially. The backscattering profile is calculated as a Fourier transform of the spectrum. This is referred to as Fourier-domain Optical Coherence Tomography (FT-OCT).
OCT is intrinsically non-invasive and exhibits great potential in in-vivo measurements, where it complements more traditional technologies, such as ultrasound imaging, by employing a different contrast mechanism and by offering higher resolution, at the expense of a much lower penetration depth. The conceptual simplicity of the OCT probes, essentially consisting of optical coupling elements between a scattering medium and an interferometer arm, leads to endoluminal and enodocavitary medicine, such as endoscopy and laparoscopy, as a natural field of application. However, other applications are known, for example, in non-surgical medicine, and in the analysis of paint layers or varnished surfaces or wood.
When a remote probe head is desired, such as, for example, in endoscopic or laparoscopic applications, the probe needs to be connected to external light sources, to spectrometers and, often, to part of the interferometer. This leads to the necessity for optical fiber tethering of the distal end of the probe. If the coupling is performed through single mode fibers, very little light is collected. If multimode fibers are used, mode mixing, dispersion, curvature losses and curvature dependence of the optical pathlength of the probe need to be taken into consideration and, if necessary, compensated. Also, it may be desirable that the probe should be front-looking, as this would simplify endoscopy or laparoscopy.
Currently most probes are side looking and employ a version of a Michelson interferometer in which an arm is directly implemented on a side-looking probe tip, see for example Guillermo J. Tearney, Mark E. Brezinski,* Brett E. Bouma, Stephen A. Boppart, Costas Pitris, James F. Southern, James G. Fujimoto, In Vivo Endoscopic Optical Biopsy with Optical Coherence Tomography, Science 276 (1997) 2037-2039. In other cases, the reference arm is implemented in the tip either through a separate reflective element or using retroreflection from the back surface of the side window, as described in Alexandre R. Tumlinson, Jennifer K. Barton, Boris Pova{hacek over (z)}ay, Harald Sattman, Angelika Unterhuber, Rainer A. Leitgeb, Wolfgang Drexler, Endoscope-tip interferometer for ultrahigh resolution frequency domain optical coherence tomography in mouse colon, Optics Express 14 (2006) 1878-1887. In both cases, in order to avoid a loss of field of view, the side window is curved and needs to be kept to a thickness of the order of 100 μm. Also as, in the state of the art, the reference arm is either separate from the sample arm or a curved reference is used, and so a complex and potentially non-scalable alignment of the probe elements is necessary.
A front looking probe has been proposed by A. M. Sergeev et al. This is described in the article In vivo endoscopic OCT imaging of precancer and cancer states of human mucosa, Optics Express 13 (1997) 433-440. This probe is based on a scanned Michelson interferometer at the proximal end of the endoscopic probe, coupled to a raster-scanning distal end. To keep the image clear from artifacts deriving from the reflection from the front window, the field of view is kept on a virtual plane well clear of the front window, and protruding a few millimeters into the free space in front of the endoscopic probe. This can make manipulation of the probe awkward.
Another fiber-based OCT system is described by K. M. Tan et al in “In-fiber common-path optical coherence tomography using a conical-tip fiber”, Optics Express 17 (2009) 2375-2384. This fiber-based solution is incompatible with windows thicker than a few tens of μm, as the window thickness detracts from the depth of field.